Generally, a point-to-multipoint service in a mobile communication system means a service provided to a plurality of user equipments by a network through a point-to-multipoint connection between the network and a plurality of the user equipments. The point-to-multipoint connection can be compared to a point-to-point connection that means a connection between a network and a user equipment or between a pair of user equipments.
Recently, many efforts have been made to provide various multimedia services via a mobile communication system, which mainly relates to a point-to-multipoint service via broadcasting or multicasting. Various names are used as a point-to-multipoint service according to an associated mobile communication system. In particular, a multimedia broadcast/multicast service (MBMS), a broadcast/multicast (BCMC) service, a multicast broadcast service (MBS) and the like are sorts of the point-to-multipoint services.
In the following description, the broadcast/multicast service as a sort of the point-to-multipoint service is mainly explained. Yet, the same idea or explanation is applicable to other sorts of point-to-multipoint services.
First of all, in a mobile communication system supporting a broadcast/muiticast (hereinafter abbreviated BCMC), multimedia data including video as well as audio is transmitted at a high data rate to mobile stations located within an area.
In order to provide a BCMC service, a packet data channel of a physical layer is able to support a high data rate. In a current mobile communication system, BCMC data are transmitted via packet data channels of a previously existing physical layer.
In a BCMC service, broadcast contents created by a base station or contents delivered from another base station are transmitted to a plurality of mobile stations within a base station area. For the contents transmission using the BCMC, a protocol is agreed in advance between a base station and a mobile station.
BCMC data is transmitted via a packet data channel. Since BCMC uses a method of transmitting data to a plurality of mobile stations from one base station, there exists no individual feedback for a reception quality from each of a plurality of the mobile stations.
For instance, in case that there exists an error in a received packet, a mobile station does not feed back such a signal as an acknowledgement (hereinafter abbreviated ACK) and a negative acknowledgement (hereinafter abbreviated NACK) to a base station.
So, in order to enable all mobile stations within a base station area to receive data with a quality exceeding a prescribed level, a base station providing a BCMC service decides a transmission data rate by considering a payload size, the number of subpackets for hybrid automatic repeat request (hereinafter abbreviated HARQ), a modulation scheme and the like.
Namely, since there is no feedback from a receiving side in a BCMC service, a base station is unable to change a transmission data rate according to a channel environment but transmits a packet at a decided data rate to all mobile stations within a base station area. So, each base station decides a data rate to have a packet error rate (hereinafter abbreviated PER) equal to or lower than a reference value for all mobile stations within its area and then transmits a packet at the decided data rate.
For instance, by considering a fading environment, an interference environment or a cell radius and the like, a base station having a good channel environment overall provides a BCMC service at a high data rate or a base station having a poor channel environment overall provides a BCMC service at a low data rate.
However, as mentioned in the foregoing description, if a BCMC service is performed after deciding a data rate for each base station, it is unable to cope with channel quality degradation due to a subsequent situation. Hence, a service quality can be degraded.